In the United States alone, there are 400 coal burning power plants representing 1,600 generating units and another 10,000 fossil fuel plants. Although coal plants are the dirtiest of the fossil fuel users, oil and gas plants also produce flue gas (combustion gases) that may include CO2, NOX, SOX, mercury, mercury-containing compounds, particulates and other pollutant materials.
Photosynthesis is the carbon recycling mechanism of the biosphere. In this process, photosynthetic organisms, such as plants, synthesize carbohydrates and other cellular materials by CO2 fixation. One of the most efficient converters of CO2 and solar energy to biomass are microalgae, often referred to herein simply as “algae,” the fastest growing photoautotrophic organism on earth and one of nature's simplest microorganisms. In fact, over 90% of CO2 fed to algae can be absorbed, mostly through the production of cell mass. (Sheehan John, Dunahay Terri, Benemann John R., Roessler Paul, “A Look Back at the U.S. Department of Energy's Aquatic Species Program: Biodiesel from Algae,” 1998, NERL/TP-580-24190; hereinafter “Sheehan et al. 1998”). In addition, algae are capable of growing in saline waters that are unsuitable for agriculture.
Using algal biotechnology, CO2 bio-regeneration can be advantageous due to the production of useful, high-value products from waste CO2. Production of algal biomass during combustion gas treatment for CO2 reduction is an attractive concept because dry algae has a heating value roughly equivalent to coal. Algal biomass can also be turned into a high quality liquid fuel which is similar to crude oil or diesel fuel (“biodiesel”) through thermochemical conversion by known technologies. Algal biomass can also be used for gasification to produce highly flammable organic fuel gases suitable for use in gas-burning power plants. (e.g., see Reed T. B. and Gaur S. “A Survey of Biomass Gasification” NREL, 2001; hereinafter “Reed and Gaur 2001”).
Approximately 40 kilocalories (167 kJ) of free energy are stored in plant biomass for every mole of CO2 fixed during photosynthesis. Algae are responsible for about one-third of the net photosynthetic activity worldwide. Photosynthesis can be simply represented by the equation:CO2+H2O→light (CH2O)+O2 where (CH2O) represents a generalized chemical formula for carbonaceous biomass.
Although photosynthesis is fundamental to the conversion of solar radiation into stored biomass, efficiencies can be limited by the limited wavelength range of light energy capable of driving photosynthesis (400-700 nm, which is only about half of the total solar energy). Other factors, such as respiration requirements (during dark periods), efficiency of absorbing sunlight and other growth conditions can affect photosynthetic efficiencies in algal bioreactors. The net result is an overall photosynthetic efficiency that can range from 6% in the field (for open pond-type reactors) to 24% in the most efficient lab scale photobioreactors.
Algal cultures also can be used for biological NOx removal from combustion gases. (Nagase Hiroyasu, Ken-Ichi Yoshihara, Kaoru Eguchi, Yoshiko Yokota, Rie Matsui, Kazumasa Hirata and Kazuhisa Miyamoto, “Characteristics of Biological NOX Removal from Flue Gas in a Dunaliella tertiolecta Culture System,” Journal of Fermentation and Bioengineering, 83, 1997; hereinafter “Hiroyasu et al. 1997”). Some algae species can remove NOx at a wide range of NOx concentrations and combustion gas flow rates. Nitrous oxide (NO), a major NOx component, is dissolved in the aqueous phase, after which it is oxidized to NO2 and assimilated by the algal cell. The following equation describes the reaction of dissolved NO with dissolved O2:4NO+O2+2H2O→4NO2−+4H+
The dissolved NO2 is then used by the algal as a nitrogen source and is partially converted into gaseous N2. The dissolution of NO in the aqueous phase is believed to be the rate-limiting step in this NOX removal process. This process can be described by the following equation, when k is a temperature-dependent rate constant:−d[NO]/dt=4k[NO]2[O2]
For example, NOX removal using the algae species Dunaliella can occur under both light and dark conditions, with an efficiency of NOx removal of over 96% (under light conditions).
Creating fuels from algal biotechnology has also been proposed. Over an 18-year period, the U.S. Department of Energy (DOE) funded an extensive series of studies to develop renewable transportation fuels from algae (Sheehan et al. 1998). In Japan, government organizations (MITI), in conjunction with private companies, have invested over $250 million into algal biotechnology. Each program took a different approach, but because of various problems addressed by certain embodiments of the present invention, none has been commercially successful to date.
A major obstacle for feasible algal bio-regeneration and pollution abatement has been an efficient, yet cost-effective, growth system. DOE's research focused on growing algae in massive open ponds as big as 4 km2. The ponds require low capital input; however, algae grown in open and uncontrolled environments result in low algal productivity. The open pond technology made growing and harvesting the algae prohibitively expensive, since massive amounts of dilute algal waters required very large agitators, pumps and centrifuges. Furthermore, with low algal productivity and large flatland requirements, this approach could, in the best-case scenario, be applicable to only 1% of U.S. power plants. (Sheehan et al. 1998). On the other hand, the MITI approach, with stricter land constraints, focused on very expensive closed algal photobioreactors utilizing fiber optics for light transmission. In these controlled environments, much higher algal productivity was achieved, but the algal growth rates were not high enough to offset the capital costs of the expensive systems utilized.
Typical conventional photobioreactors have taken several forms, such as cylindrical or tubular bioreactors, for example as taught by Yogev et al. in U.S. Pat. No. 5,958,761. These bioreactors, when oriented horizontally, typically require additional energy to provide mixing (e.g., pumps), thus adding significant capital and operational expense. In this orientation, the O2 produced by photosynthesis can become trapped in the system, thus causing a reduction in algal proliferation. Other known photobioreactors are oriented vertically and agitated pneumatically. Many such photobioreactors operate as “bubble columns,” as discussed below. Some known photobioreactor designs rely on artificial lighting, e.g. fluorescent lamps, (such as described by Kodo et al. in U.S. Pat. No. 6,083,740). Photobioreactors that do not utilize solar energy but instead rely solely on artificial light sources can require enormous energy input.
Many conventional photobioreactors comprise cylindrical algal photobioreactors that can be categorized as either “bubble columns” or “air lift reactors.” Bubble columns are typically translucent large diameter containers filled with algae suspended in liquid medium, in which gases are bubbled at the bottom of the container. Since no precisely defined flow lines are reproducibly formed, it can be difficult to control the mixing properties of the system which can lead to low mass transfer coefficients, poor photomodulation, and low productivity. Air lift reactors typically consist of vertically oriented concentric tubular containers, in which the gases are bubbled at the bottom of the inner tube. The pressure gradient created at the bottom of this tube creates an annular liquid flow (upwards through the inner tube and downwards between the tubes). The external tube is made out of translucent material, while the inner tube is usually opaque. Therefore, the algae are exposed to light while passing between the tubes, and to darkness while passing in the inner tube. The light-dark cycle is determined by the geometrical design of the reactor (height, tube diameters) and by operational parameters (e.g., gas flow rate). Air lift reactors can have higher mass transfer coefficients and algal productivity when compared to bubble columns. However, control over the flow patterns within an air lift reactor to achieve a desired level of mixing and photomodulation can still be difficult or impractical. In addition, because of geometric design constraints, during large-scale, outdoor algal production, both types of cylindrical-photobioreactors can suffer from low productivity, due to factors related to light reflection and auto-shading effects (in which one column is shading the other).
Another problem in utilizing conventional algal culture technology and conventional commercially available algal cultures for pollution mitigation on-site in photobioreactors is that commercially available cultures of species suitable for pollutant abatement are typically poorly suited to grow under light and other conditions that the algae will experience in operation. This can lead to a failure of the culture and photobioreactor system and/or to infiltration of the photobioreactor by parasitic organisms better suited to growth under the prevailing conditions, but that are unable or poorly suited to abatement of pollutants, such as NOx and SOx.